constitutionally legit control: enact permits and charge a % of the take.
Historic irony dept. Way back when BART directors were appointees (and a favor for connected people) one opined at a meeting that BART itself should install some sort of music system to relieve boredom while waiting for trains (obviously not understanding that headways should be less that a 3 min pop song) noting that the music could be themed by cultural holidays such as Irish music in March...

While busking is certainly a long tradition,if you are waiting to meet someone at a BART station, the repetitive music can be annoying